In recent years, gallium nitride-based compound semiconductors have become of interest as materials for producing a light-emitting device which emits light of a short wavelength. Generally, a gallium nitride-based compound semiconductor is grown on a substrate made of an oxide crystal such as a sapphire single crystal, a silicon carbide single crystal, or a Group III-V compound single crystal, through a method such as metal-organic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE).
At present, the crystal growth method that is most widely employed in the industry includes growing a semiconductor crystal on a substrate such as sapphire, SiC, GaN, or AlN, through metal-organic chemical vapor deposition (MOCVD). Specifically, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer are grown on the aforementioned substrate, in a reactor tube, by use of a Group III organometallic compound and a Group V source gas and at a temperature of about 700° C. to about 1,200° C. After the growth of the above layers, a negative electrode is formed on the substrate or the n-type semiconductor layer, and a positive electrode is formed on the p-type semiconductor layer, whereby a light-emitting device is fabricated.
Conventionally, such a light-emitting layer is formed from InGaN whose composition is controlled so as to adjust the light emission wavelength. The active layer is sandwiched by layers having a bandgap higher than that of InGaN, thereby forming a double-hetero structure, or is incorporated into a multiple quantum well structure on the basis of the quantum well effect.
In a gallium nitride-based compound semiconductor light-emitting device having a light-emitting layer of a multiple quantum well structure, when the thickness of a well layer is adjusted to 2 to 3 nm, satisfactory output is attained, but a problematically high operating voltage is required. In contrast, when the thickness of the well layer is 2 nm or less, the operating voltage is lowered, but the satisfactory output, i.e., high-efficiency light emission, cannot be attained.
U.S. Patent Application Publication US2003/0160229A1 has proposed a gallium nitride-based compound semiconductor light-emitting device comprising a light-emitting layer having a multiple quantum well structure, where a thickness variation is imparted to the active well layer so as to obtain high-efficiency light emission.
Also, Japanese Patent No. 3,660,446 discloses a nitride semiconductor light-emitting device having a quantum well structure where the well layer has a non-uniform thickness. According to this patent publication, in the active layer of the quantum well structure, a very thin nitride semiconductor layer or a metal layer, having in-plane thickness distribution and in-plane crystallinity distribution, is provided below a well layer and the well layer is grown thereon, whereby the well layer is selectively grown, the active layer becomes a quantum dot, the light emission efficiency is greatly enhanced, and a highly reliable nitride semiconductor device can be realized. Furthermore, in this patent publication, the device reliability is enhanced by varying the thickness of the layer stacked below the well layer to change the film quality of the well layer.
However, studies by the present inventors have revealed that in a gallium nitride-based compound semiconductor light-emitting device comprising a light-emitting layer having a multiple quantum well structure, when the thickness of the well layer is made non-uniform, the light emission intensity disadvantageously deteriorates due to aging.